Engagement of the T-cell antigen receptor (TCR) complex triggers a cascade of protein tyrosine phosphorylation events that are crucial for triggering of the T-cell activation program. While the proximal protein tyrosine kinases (PTKs) responsible for the initiation of this phosphorylation cascade are well-defined, the downstream targets of these PTKs are incompletely understood. The investigator's laboratory has cloned a novel Fyn-binding protein, termed Fyb, which is phosphorylated in response to TCR ligation. The Fyb protein contains multiple tyrosine phosphorylation sites, a putative nuclear localization signal, and an SH3-like domain. Fyb expression is restricted to myeloid cell lines and T cells. Immunofluorescence studies indicate that Fyb is widely distributed throughout T cells, including a significant level of staining in the nucleus. Perhaps most importantly, phosphorylated Fyb binds to the SH2 domains of Fyn and SLP-76, two candidate signal transducers for the TCR complex. Over-expression of Fyb in T cells leads to augmented IL-2 secretion following TCR cross-linkage, suggesting that Fyb is a positive effector of activation-induced cytokine production. The goals of the project described in this application are: (1) to identify the PTK(s) responsible for the phosphorylation of Fyb in response to TCR ligation, (2) to identify the mechanism by which Fyb augments IL-2 production, (3) to understand the biochemical basis and functional importance of the cytoplasmic versus nuclear localization of Fyb, and (4) to examine the impact of Fyb over-expression on intra-thymic T-cell development using transgenic mouse models.